Merge branch '6.3' into 6.4

* 6.3:
  Minor tweak
  Adding `importmap_polyfill` (second attempt)

Author: javiereguiluz

frontend/asset_mapper.rst

@@ -289,7 +289,8 @@ Preloading and Initializing "app.js"
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 In addition to the importmap, the ``{{ importmap() }}`` Twig function also renders
-an `ES module shim`_ and a few other things, like a set of "preloads":
+an `ES module shim`_ (see the :ref:`config-importmap-polyfill <polyfill config>`) and
+a few other things, like a set of "preloads":
 
 .. code-block:: html
 
@@ -1027,6 +1028,21 @@ This is a list of glob patterns that will be excluded from the asset map:
 You can use the ``debug:asset-map`` command to double-check that the files
 you expect are being included in the asset map.
 
+.. _config-importmap-polyfill:
+
+``framework.asset_mapper.importmap_polyfill``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Configure the polyfill for older browsers. Default is `ES module shim`_. You can pass
+any URL to be included, or ``false`` to disable the polyfill.
+
+.. code-block:: yaml
+
+    framework:
+        asset_mapper:
+            importmap_polyfill: false # disable the shim ...
+            # importmap_polyfill: 'https://...' # ... or pass some custom URL
+
 ``framework.asset_mapper.importmap_script_attributes``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~